1. Field of the Invention
This invention relates to the field of semiconductor device manufacture, and in particular, to a method of manufacturing semiconductor devices such that the devices are optically inspectable and an optically inspectable semiconductor device.
2. Description of Related Art
During the semiconductor manufacturing process layers of material are deposited on the substrate to form electrical elements thereon. It is necessary and desirable to detect defects which are introduced in a particular process or formation of an electrical device so that continual processing may be avoided prior to correction or removal of the defective wafer in the production line. Optical inspection of the semiconductor device during multiple stages of manufacture enhance the quality of the device as a whole.
Generally, optical inspection tools operate by emitting a beam of light through the transparent or near-transparent substrate or device. Defects within the device reflect light back to a detector located on the inspection tool. It is difficult to discriminate those defects occurring in the most recent stage of manufacturing from those defects which occurred in prior stages.
One example where prior art methods of optical inspection are inadequate is in processing silicon-on-insulator (SOI) wafers produced by oxygen ion implantation. In oxygen ion implantation, a high dose of oxygen is implanted into a silicon wafer. The wafer is subsequently annealed at high temperature whereby the oxygen coalesces to form a silicon dioxide insulator layer buried beneath the silicon surface. A thin single crystal silicon layer, the SOI layer, is then formed above the silicon dioxide insulator.
Annealing the silicon wafer and oxygen produces a number of silicon inclusions located within the bottom of the silicon dioxide insulator layer. They are on the magnitude of 10.sup.4 or 10.sup.6 /cm.sup.2. These silicon inclusions can be seen using optical inspection tools.
Further processing on an SOI wafer typically involves some form of isolation to separate the electrical elements thereon. A common choice is shallow trench isolation (STI). FIGS. 1 and 2 show a shallow trench isolation manufactured in accordance with prior art methods.
FIG. 1 shows a silicon wafer 3 following oxygen ion implantation to form the buried silicon oxide insulator layer 2. The thin single crystal silicon layer is represented by the number 1. Near the bottom of the insulator layer 2 are a plurality of silicon inclusions 4 formed as a result of the annealing process.
In forming the STI in the SOI wafer, a typical process comprises depositing a sacrificial film such as an oxide and nitride followed by patterning and etching a trench 25 to the buried insulator layer 4. A thin thermal oxide 8 is grown in the walls of trench 25. A second insulating layer 12 such as tetraethylorthosilicate (TEOS), is deposited conformally on the wafer to fill trench 25. Any excess of the second insulating layer is planarized.
Assume the silicon layer of the SOI wafer is 200 nm thick and the optical absorption of silicon at a wavelength of 500 nm is 1.3E-3 [1/nm]. An optical inspection tool such as the KLA 2138 manufactured by KLA/Tencor, 160 Rio Robles, San Jose, Calif. 95134, operating at 490 nm through 650 nm would be able to see the silicon inclusions.
A light having a wavelength of 500 nm emitted by the inspection tool traverses through the 200 nm of silicon, down through the SOI layer, is scattered by the inclusions, and the scattered light passes back through the SOI layer to the detector. The total path length in the silicon is 400 nm. The 500 nm light is attenuated by I/I.sub.0 =e.sup.-(1.3E-3)(400) =0.6, a rather small attenuation of the light emitted by the inspection tool.
However, one could prevent the inspection tool from seeing defects below the SOI layer by using light with a short enough wavelength that it is substantially absorbed by the SOI layer. An example would be to use a 365 nm wavelength. The absorption of silicon at 365 nm is 0.1[1/nm], thus in passing through 200 nm of silicon, the light is attenuated to I/I.sub.0 =e.sup.(0.1)(400) =4.3E10.sup.-18 or substantial absorption of the light. Unfortunately, the defects below the STI region will still be detectable by the inspection tool since the STI region is filled with oxide which is transparent. Defects occurring in subsequent processing must be discriminated against the defects seen below the STI region making inspection difficult.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of manufacturing and optically inspecting silicon wafers wherein defects created by subsequent processing can be distinguished against defects created in prior level processing.
It is another object of the present invention to provide a method of manufacturing and optically inspecting SOI wafers wherein defects created by subsequent processing can be distinguished against defects created in prior level processing.
A further object of the present invention is to provide an optically inspectable semiconductor device.
It is yet another object of the present invention to provide an optically inspectable SOI wafer having an STI.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.